The present invention relates to a system for controlling ignition timing of an internal combustion engine such as an automotive engine, and more particularly to a system for advancing the ignition timing at acceleration of the engine.
In a conventional ignition timing control system, the top dead center is detected by a crank angle sensor and the ignition timing is set in advance of it. A period of time for a dwell angle, through which current passes in a primary winding of an ignition system, is further taken account of to determine the timing for starting the current to flow in the primary winding. At the advanced ignition timing, the current is cut off so that a spark plug generates a spark by the high voltage produced in a secondary winding. The ignition timing is set close to a maximum ignition timing MBT shown in FIG. 3 to obtain a maximum torque. However, when the ignition timing is advanced too close to the MBT, the auto-ignition of highly compressed end-gas occurs. Thus, the pressure in the combustion chamber rapidly increases, causing knocking of the engine. Accordingly, there is a limit timing A.sub.1 for limiting the advance of the ignition timing. Namely, in order to prevent knocking, the timing is set at the timing A.sub.1. However, the ignition timing is further retarded from the timing A.sub.1 to a ignition timing A.sub.2 by a surplus time .alpha. as a safeguard against knocking. The time .alpha. is provided in consideration of timing variances caused by manufacturing tolerances of the engine, retardation of the limit timing A.sub.1 by deterioration of the ignition system, and other factors.
However, as shown in FIG. 3, torque inevitably drops from T.sub.1 to T.sub.2 because of the retardation of the time .alpha.. Particularly a sufficient acceleration characteristic of the engine can not be obtained. Thus, it is necessary to adjust the timing for the acceleration.
Japanese Patent Laid Open No. 59-145364 discloses an ignition timing control system wherein the ignition timing is advanced when the rapid acceleration of the vehicle is detected by throttle opening degree and changing rate thereof. However, since the acceleration is determined dependent on the operation of the throttle valve only, the detected acceleration does not necessarily coincide with the actual acceleration of the vehicle.